Multiple motor operation using solar power

ABSTRACT

A solar powered system controls multiple motors directly from a single solar power source with a single variable frequency drive (VFD). A control system selectively connects and disconnects individual motors as solar power varies so that the available power can be used effectively and some of the motors can be kept operating rather than shutting down the entire system. The control system includes a detection circuit for detecting the solar array voltage, and a selection circuit for providing shutoff control signals to individual motors based on the solar array voltage and preselected motor shutoff priority criteria.

RELATED APPLICATIONS

This application claims priority of Provisional Application Ser. No.60/723,268 filed Oct. 3, 2005.

BACKGROUND OF INVENTION

1. Field of the Invention

The invention relates generally to solar powered systems, and moreparticularly to the operation of multiple motors using solar power.

2. Description of Related Art

Solar power is being increasingly used as an alternate energy source.One application is to provide power to operate motors. Most motors areAC motors. The DC output of a solar panel array is converted to AC powerby a variable frequency drive (VFD). In many applications, it isdesirable to drive an array of multiple motors from a single VFDconnected to a solar power source.

Controlling multiple motors from a single variable frequency drive (VFD)has been done but always from an AC grid voltage which is a stable powersource capable of supplying the required power. When used with solarpower, caution must be taken as the power supply is not stable but avarying power supply. Multiple motor operation under these conditionscan result in starved power to the motors, resulting in motor damage.Under these conditions, there may be sufficient power to operate onlysome of the motors. Thus a system is desired in which motors can beselectively switched in and out of operation, depending on the availablepower, so that the most essential motors keep operating, instead ofshutting down the entire system.

It is important for solar powered motor systems to alter or control thepower to the motors to prevent starving, commonly referred to as“brownout conditions”. The present invention covers the requirement ofvarying power to the motors as a function of solar voltage or power.

An important aspect of these solar powered motor systems is motor inrushwhich can result in shutdown of the entire VFD drive if not controlled.These inrush currents last for approximately 2-3 cycles, but are anoverload current. One way to avoid shutdown is to stagger the turn on ofeach motor so that the VFD does not reach an overload condition. Anotherway is to size a drive capable of handing the inrush currents for allmotors. The latter is most expensive and dependent on motor inrushcurrents. This is true for AC grid operated designs as well. Thus it isdesirable to provide a system to control the turn on and turn off ofeach of the motors to prevent an overload shutdown condition of theentire system.

SUMMARY OF THE INVENTION

This invention is a solar powered system for controlling multiple motorsdirectly from a single solar power source with a single variablefrequency drive (VFD). A control system selectively connects anddisconnects individual motors as solar power varies so that theavailable power can be used effectively. The invention detects overloadcurrents and staggers motor operation in the event of an overloadcondition. Thus some of the motors can be kept operating rather thanshutting down the entire system. The invention also detects solar arrayvoltage and automatically adjusts motor frequency for optimum powerperformance.

In one aspect of the invention, the control system includes a detectioncircuit for detecting the solar array voltage, and a selection circuitfor providing shutoff control signals to individual motors based on thesolar array voltage and preselected motor shutoff priority criteria.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a block diagram of a solar powered multiple motor system witha control system of the invention.

FIG. 2 is a block diagram showing the connection of the control systemto individual motors.

FIG. 3 is a schematic diagram of a control logic circuit of theinvention.

FIG. 4 is a block diagram of the control system of the invention.

DESCRIPTION OF THE INVENTION

As shown in FIG. 1, a solar powered multiple motor system 10 of theinvention utilizes a solar array 12 as a source of power. The DC outputof solar array 12 is input into a variable frequency drive (VFD) 14 toconvert the DC power to AC power. The AC output of VFD 14 drives motorarray 16, which comprises a plurality (N) of individual motors. Controlsystem 18 is connected between solar array 12 and motor array 16 tocontrol the individual motors therein on the basis of the power producedby solar array 12.

As shown in FIG. 2, a plurality of motors 20, 21, 22, 23 (Motor 1 . . .Motor N) of the motor array 16 are connected in parallel to bus 24 ofVFD 14. Control system 18 is connected to each of the motors 20, 21, 22,23. Control system 18 selectively turns the individual motors 20, 21,22, 23 off and on depending on the available solar power, as determinedfrom the solar array voltage.

In accordance with the invention, a control system is provided whichmonitors the solar array voltage. With the solar array at maximum power,all motors are powered by the solar driven VFD. As the solar array powervaries, the solar voltage varies with the variation in solar power. Fordecreasing solar voltage, the solar power is decreasing and the motorfrequency is adjusted for the change in solar power. There is acondition where the solar power is not capable of supporting all themotors and continued use will result in an automatic shutdown. This isnot a fault that can result in motor damage but a controlled shutdownfor all motors, which may have significant adverse consequences.

If selected motors are required to run under lower solar power whichcannot sustain operation of all the motors, the control systemselectively shuts down nonessential motors so that the remaining motorscan stay in operation. This can be done by shutting down individualmotors or a bank of motors. As the power increases, additional motorscan be turned back on. The flexibility of the logic circuit allows forthis operation. For example, there may be 10 motors connected to aspecific VFD drive all in parallel. The logic allows selection of onemotor or any number of motors for controlled shutdown. The user canselect which motors are the most important and which are the leastimportant, and at what solar voltage (power) level each motor drops outor comes back on.

FIG. 3 shows a logic control circuit 30 which is used to select orsequence an orderly shutdown or control of selected motors. A solararray voltage sensing resistor network 32 is provided to detect changesin solar voltage, which is representative of changes in solar power.Network 32 is made up of resistors R1, R2, R3, R4, R5 connected inseries between solar array positive line 33 and solar array negativeline 34. The voltage drops across each of the resistors providedifferent voltage level inputs V1, V2, V3, V4 to the positive inputs ofcorresponding comparators 35, 36, 37, 38 whose outputs are connected toa corresponding motor 20, 21, 22, 23. A reference voltage Vref1, Vref2,Vref3, Vref4 is input into the corresponding negative inputs of thecomparators 35, 36, 37, 38.

For each comparator, its output will be high as long as Vn>Vrefn. WhenVn drops to Vrefn or less, the output of the comparator will go to zero(low). The output of each comparator controls the associated motor. Whenthe comparator output is high, the motor is turned or kept on; when thecomparator output is low, the motor is turned or kept off.

The input voltages V1, V2, V3, V4 are proportional to the solar arrayvoltage (and power). For example, the resistors may be chosen so that atfull array voltage, e.g. 300 V, the input voltages are 10 V, 8 V, 6 V,and 4 V. When the array voltage drops a certain percent, e.g. 10%, thesevoltages will also drop the same percent, e.g. 10%. The referencevoltages can be chosen so that each comparator switches from high to lowat a particular array voltage level. The comparator connected to a motorthat is deemed more essential and should be kept on as long as possiblewill have a reference voltage that is much lower than the maximum inputvoltage (Vnmax). Then that motor will not be shut off until the arrayvoltage drops very low, and will come back on sooner. The comparatorconnected to a motor that is deemed less essential and can be shut offsooner will have a reference voltage that is much closer to the maximuminput voltage. Then that motor will shut off sooner, and will come backon later. For example, if M1 is very important and M2 is not veryimportant, then Vref1 can be set at 50% of V1max while Vref2 can be setto 90% of V2max. Then when the solar power drops 10%, M2 will be shutoff, leaving more power for the rest of the motors, while M1 will not beshut off until the array power drops 50%. If the whole motor array isshut off, M1 will come back on when the power reaches 50% while M2 willnot come on until array power reaches 90%.

Thus levels can be set such that a sequence of motors can begin to beshut off when predetermined threshold voltages are reached. The controlsystem can be set to turn off various motors, highest power rating orthe lowest, based on whatever selection criteria the user chooses, topreserve the array power for the balance of motors to run. When thesolar array voltage reaches a minimum level, no more solar power isavailable for any motor, and the control logic will shut down all themotors.

Logic circuit 30 is a particular embodiment of control system 18.However, control system 18 can be implemented in any embodiment based onthe principles of the invention. For example, the comparators shown inFIG. 3 can be replaced with low logic relays for isolation if needed.

FIG. 4 shows a more general control system 40 for carrying out theinvention. Control system 40 has a sensing circuit 42 coupled to aselection circuit 44. Sensing circuit 42 senses the solar array voltage.Selection circuit 44 is connected to the sensing circuit 42 and providesshutoff control signals to individual motors based on changes in thesolar array voltage and preselected motor shutoff criteria.

Changes and modifications in the specifically described embodiments canbe carried out without departing from the scope of the invention whichis intended to be limited only by the scope of the appended claims.

1. A control system for operating a motor array from a solar array,comprising: a sensing circuit for sensing the solar array voltage; aselection circuit connected from the sensing circuit to the motor array,the selection circuit providing shutoff control signals to individualmotors in the motor array based on changes in the solar array voltageand preselected motor shutoff priority criteria.
 2. The control systemof claim 1 wherein the sensing circuit comprises a resistor network. 3.The control system of claim 2 wherein the resistor network comprises aplurality of resistors in series, each resistor producing a voltage dropproportional to the solar array voltage.
 4. The control system of claim1 wherein the selection circuit comprises a plurality of comparators,each comparator having one input connected to the sensing circuit forreceiving a signal based on the solar array voltage and a second inputconnected to a reference voltage determined by the preselected motorshutoff priority criteria, the output of each comparator being connectedto an associated motor.
 5. The control system of claim 3 wherein theselection circuit comprises a plurality of comparators, each comparatorhaving one input connected to an associated voltage drop in the resistornetwork and a second input connected to a reference voltage determinedby the preselected motor shutoff priority criteria, the output of eachcomparator being connected to an associated motor.
 6. Apparatuscomprising: a solar array for producing DC power; a variable frequencydrive (VFD) connected to the solar array for converting the DC power toAC power; a motor array comprising a plurality of individual motorsconnected to the VFD; a control system according to claim 1 connectedbetween the solar array and the individual motors of the motor array forselectively shutting off individual motors as the solar voltage changesaccording to the preselected motor shutoff priority criteria.
 7. Theapparatus of claim 6 wherein the sensing circuit comprises a resistornetwork.
 8. The apparatus of claim 7 wherein the resistor networkcomprises a plurality of resistors in series, each resistor producing avoltage drop proportional to the solar array voltage.
 9. The apparatusof claim 6 wherein the selection circuit comprises a plurality ofcomparators, each comparator having one input connected to the sensingcircuit for receiving a signal based on the solar array voltage and asecond input connected to a reference voltage determined by thepreselected motor shutoff priority criteria, the output of eachcomparator being connected to an associated motor.
 10. The apparatus ofclaim 8 wherein the selection circuit comprises a plurality ofcomparators, each comparator having one input connected to an associatedvoltage drop in the resistor network and a second input connected to areference voltage determined by the preselected motor shutoff prioritycriteria, the output of each comparator being connected to an associatedmotor.
 11. A method for controlling the individual motors of a motorarray operated from a solar array, comprising: sensing the solar arrayvoltage; selecting individual motors to shut off based on changes in thesolar array voltage and preselected motor shutoff priority criteria. 12.The method of claim 11 wherein selecting individual motors to shut offis performed by comparing a sensed voltage proportional to the arrayvoltage to a reference voltage.
 13. The method of claim 12 wherein thereference voltage is determined to shut off an individual motor when thesolar voltage reaches a preselected level.
 14. The method of claim 11further comprising shutting off enough individual motors so thatavailable solar power is sufficient to operate the remaining motors.